THE COMPOUND EYES OF INSECTS. 265 



Pseudocone eye, which has only a semi-fluid cone ; and the 

 Acone eye, that has none at all. From what has been said, 

 it seems reasonable to assume that the retina of the insect 

 lies in contact with its cornea, and that we have no right 

 to assume that the cones have any different function from 

 that which they possess in the human eye, although we 

 admit that at present their immediate use is not positively 

 known. The next question that forces itself upon the mind 

 is this : If no lens is present, how are the images focussed 

 upon the insect's retina 1 The generally accepted answer is, that 

 the corneal facets are lenses ; but if this be true, how can the 

 images be focussed, seeing that the cornea is a rigid body, not 

 capable of movement or of compression ? The entomologist 

 usually replies, that it is more than probable that the image 

 is always blurred, and that the insect has no defined image save 

 perhaps at one special distance, or of objects at what is termed 

 " infinity." There is, however, another matter to which atten- 

 tion will be now directed, and to which a suitable reply seems 

 impossible. If the corneal lenses be truly a reality, why do 

 they not show signs of strain with polarised light, like any other 

 lens would do if pressed as strongly by the cover-glass as the 

 cornea has to be to get it sufficiently fiat for the use of the 

 microscopist ? Any lens with such a pressure would show 

 the most positive signs of "strain." Why do the facets present 

 nothing of the kind ? To the thoughtful mind it would seem 

 difficult to believe in any theory which is threatened with such 

 unanswerable questions. 



Some four or more years ago I was searching for secondary 

 markings in a diatom with well-marked cellular structure, and 

 was surprised to see a little marking in the centre of each of 

 the numerous small holes in the object. As these markings 

 were so regularly visible in every hole, it was suspected that 

 the effect must be due to some subjective influence. It was ulti- 

 mately traced to the presence of a little piece of foreign matter 

 in the cell containing green fluid, which was being employed as 



